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FRITZ!Box reports "Interworking error"

You cannot make calls with a FRITZ!Fon registered with a FRITZ!Box, or it may not be possible to call only certain numbers. The following messages are displayed:

  • On the telephone's display: "Interworking error"
  • In the event log of the FRITZ!Box: "Internet telephony with [number] failed. Cause: Server Internal Error (500)"

Simply proceed as described below. After each measure, check whether the problem is solved.

1 Restarting the FRITZ!Box

  1. Click "System" in the FRITZ!Box user interface.
  2. Click "Backup" in the "System" menu.
  3. Click on the "Restart" tab.
  4. Click the "Restart" button.

2 Reconfiguring telephone numbers

  1. Click "Telephony" in the FRITZ!Box user interface.
  2. Click "Telephone Numbers" in the "Telephony" menu.
  3. Click the (Delete) button for the telephone number.
  4. Reconfigure the telephone number and select your internet telephony provider during setup.
  5. If you are unable to register the telephone number after setup, contact your provider to have them check your telephone number and login information.

3 Disabling HD telephony

  1. Click "Telephony" in the FRITZ!Box user interface.
  2. Click "Telephony Devices" in the "Telephony" menu.
  3. Click the (Edit) button for the respective telephone.
  4. Click on the "Telephony Device Features" tab.
  5. Select the option "HD disabled" from the drop-down list in the section "HD Telephony".
  6. Click "OK" to save the settings.

The FRITZ!Box is now fully configured for making calls over the internet. If you still have problems, your internet telephony provider may be experiencing technical difficulties.